The present invention relates generally to a process for delignifying and bleaching lignocellulosic pulps and, more particularly, to a novel process for delignifying and bleaching chemical wood pulps with manganese.
It is known in the bleaching art that when lignocellulosic pulps are bleached with oxygen in an alkaline medium, the presence of manganese in catalytic amounts will confer certain benefits. For example, Minor and Landucci have reported (see International Pulp Bleaching Conference, 1973, Vancouver, B.C., pg. 83) that the rate of oxygen-alkali delignification of southern pine groundwood is accelerated by the addition of 0.01% manganese, but only a small effect was noted with a kraft pulp having a 7% lignin content. Gilbert et al. (TAPPI, 56(6), p. 95, 1973) found that while additions of only 10 ppm of manganese during oxygen-alkali bleaching of cotton linters resulted in viscosity losses, the use of increased amounts of manganese, i.e., 60 ppm, resulted in viscosity improvements.
Japanese Laid-Open Specification No. 49(74)-503 discloses that the presence of catalytic amounts of manganese, i.e., 0.2 weight percent, during oxygen bleaching inhibits carbohydrate degradation, as shown by improved strength properties. It also discloses that residual precipitated manganese compounds, which cause the pulp to have a pink coloration, can be removed by washing with dilute sulfuric acid.
It thus becomes apparent that the role of manganese, when employed in catalytic amounts in oxygen bleaching, is to react with the oxygen and alkali to form the insoluble compound, manganic hydroxide. Manganic hydroxide is not consumed stoichiometrically in reactions with lignin, but, rather, as a catalyst, it promotes or accelerates the reaction between oxygen and lignin, while also preventing excessive losses in viscosity.
As bleaching agents per se, manganese compounds have received little recognition. While Bradley et al., U.S. Pat. No. 1,795,757, discloses a bleaching system based on the use of manganate ion, +5, and permanganate ion, +7, which represent the two highest stable oxidation states of manganese, it must be used in conjunction with hypochlorite. Hypochlorite, being an extremely strong oxidant, contributes significantly to the bleaching potential and also facilitates the regeneration of Mn.sup.+5 and Mn.sup.+ 7. It also discloses an SO.sub.2 or sulfurous acid wash after bleaching, but solely as a means of removing the manganese compounds from the fibers to avoid discoloration.